Myocardial electrical impedance is correlated to the vitality of the myocardial tissue. It is well established in the literature that myocardial impedance increases by as much as 200 percent of baseline value with regional and global ischemia, edema, pathologic tissue ultrastructural changes ATP depletion and lactate accumulation. When these pathophysiologic conditions are reversed before permanent tissue damage is incurred, the myocardial electrical impedance returns to baseline values. The exact etiology of these electrical impedance changes is unknown.
Myocardial electrical impedance has both a resistive and a capacitive reactive component. It has been shown that the phase angle between the real and imaginary part of the myocardial electrical impedance at a frequency of 5 KHz and, to a lesser extent, the real part of the myocardial electrical impedance at 200 Hz exhibit a specific characteristic rise during ischemia.
The literature clearly indicates that myocardial electrical impedance has value as a diagnostic tool. Myocardial electrical impedance can identify severe but reversible ischemic injury. The variable has potential use in measuring the viability of the myocardium during cardioplegia and determining the need for and type of resuscitation following coronary artery bypass graft surgery.
It is hypothesized that myocardial electrical impedance may also be useful in diagnosing tissue rejection by the immune system following cardiac transplant surgery. Cardiac tissue rejection results in myocardial edema and other conditions that have previously been shown to affect cardiac tissue impedance. Currently, the condition of the transplanted cardiac tissue is monitored by routine histopathologic biopsies taken from the endocardium. The results from this procedure often take a day to obtain whereas myocardial electrical impedance can be measured in a matter of minutes. The potential benefit of using myocardial impedance rather than histopathologic biopsies is that tissue rejection could be diagnosed and treated much earlier which may result in reversing the rejection process and preserving the transplanted heart.
Myocardial electrical impedance has been measured between 0 and 10 KHz using techniques requiring both two and four electrodes to be attached to the heart tissue. One two-electrode technique, demonstrated for instance by Jones et al. (1928), employs a Wheatstone bridge network consisting of four impedances connected in a circular series configuration. Two of the impedances are discrete components of known fixed value. The third impedance is typically composed of a discrete calibrated variable resistor and capacitor connected in parallel. The fourth impedance is the tissue to be measured and is connected to the Wheatstone bridge via the two electrodes. A direct current or sinusoidal alternating current signal generator at a given desired frequency is connected to opposite corners of the bridge and a detector such as headphones or an oscilloscope connected to the opposing corners of the bridge. The impedance of the tissue is determined by adjusting the variable resistor and capacitor to achieve a null in the detector and then reading the values of resistance and capacitance from the calibrated components.
Another two-electrode technique, referred to by Garrido et al. (1983) and called a myocardial electrical impedance meter, apparently measures myocardial electrical impedance at one specific frequency. That frequency may be 0 Hz.
The four-electrode technique has been the method most often used to measure tissue impedance in the recent research literature. This technique, described for instance by Ellenby et al. (1987), employs a linear array of four equally spaced electrodes attached to the myocardial tissue. A known electrical current is impressed between the outer two electrodes and the resulting voltage drop is measured between the inner electrodes. The tissue impedance is calculated from Ohm's Law by dividing the voltage by the current. This technique has been used to measure tissue impedance at frequencies between 0 and 10 KHz. The frequency at which the impedance is measured is determined by the frequency of the impressed current signal. This method has been used to extract the resistive and capacitive reactive components of the impedance as well as the phase of the impedance at a given frequency.
These past techniques for measuring myocardial electrical impedance have several disadvantages which make them impractical for clinical use. The two-electrode techniques cause electrode-tissue polarization. This phenomenon has been shown to cause drift and general inaccuracy in the electrical impedance measurement. The four-electrode technique requires that a more complicated, less clinically feasible electrode montage be placed on the myocardium in order to perform the measurement. Finally, none of these previous techniques generates an electrical impedance spectrum, but rather only generates an impedance at one particularly frequency.
In view of past research in myocardial electrical impedance and the potential clinical utility of myocardial impedance it is an object of the invention to provide a myocardial electrical impedance spectrometer having the following ideal design objectives: a) the spectrometer should determine myocardial tissue electrical impedance over a certain frequency range of interest rather than simply a meter which measures impedance at one particular frequency since conditions such as ischemia produce characteristic complex impedance changes at specific frequencies; b) the apparatus should evaluate the complex electrical impedance rather than just the modulus or magnitude of the impedance since the phase angle of the impedance at certain frequencies has shown to have characteristic changes with ischemia; c) the apparatus should evaluate the myocardial electrical impedance spectrum in a relatively short period of time in order to promote early diagnosis and early effective treatment of pathologic heart conditions; and d) the apparatus should have an uncomplicated electrode interface with the heart and, where possible, make use of a preexisting electrode interface.